Chemical recovery boilers perform two basic functions: burning organics to make steam for various mill processes and recovering the inorganic chemicals used in the pulping process. The processed inorganic chemicals or “smelt” from the recovery boiler are collected at the bottom of the furnace and are discharged through dedicated openings in the lower furnace into a main dissolving tank. The main dissolving tank is filled with waste water from a lime mud washing process, which is also known as weak wash. The molten smelt, with temperatures as high as 1500° F. must be broken into small droplets using jets of steam or weak wash before the molten smelt enters the main dissolving tank. If the smelt is not properly shattered, there can be an explosive reaction between the water in the weak wash in the dissolving tank. In the dissolving tank, the smelt is dissolved into either water, during start-up, or weak wash to produce green liquor.
There are two distinct methods for smelt to be collected and discharged from the lower furnace. A decanting hearth design is where the furnace bottom is flat and an inventory of smelt collects in the lower furnace. As the level rises the smelt flows into sloped discharged chutes, known as smelt spouts, to the shatter jets and dissolving tank. The second design has a sloped furnace floor where the smelt flows by gravity towards the smelt spouts. In either design, blockage of these spouts can require immediate corrective actions on the part of the outside operator. Typically a small percentage of the smelt freezes to the external surface of the water cooled trough due to cooling water supply temperatures being in the 150° F. to 180° F. range. This is common and is a form of protection for the metals used in the spout. However, the liquid level or tide line in the trough may also freeze due to ambient temperatures and tends to crust over blocking the free flow of smelt.
Normal operation of the spout enclosures requires routine observation and maintenance that includes manual manipulation of the smelt within the smelt spout using a long rod (known as “rodding”) to insure smelt flow from the spout. Under normal operation, access door(s) on a spout enclosure are closed to keep the effects of tank drafts to a minimum and to prevent re-oxidization of the smelt flowing off of the spout. When the spout is rodded, the access doors are opened and several things occur. The uniformity of the stream of smelt flowing off of the spout is commonly disturbed due to the rodding and/or drafts induced by either the scrubber vent fan or the natural draft of the vent stack. This disturbed flow can negatively impact the effectiveness of the shatter jet spray which can result in either minor explosions from the dissolving tank or may deposit materials on the lower portions of the smelt spout enclosure. Other factors may also add to the building of smelt accumulations on the side walls of the smelt spout enclosures.
The use of fluid washing inside the smelt spout enclosures has been common over the last several decades to combat the accumulation of smelt on the side walls of the smelt spout enclosures. Such washing typically includes the use of a wash header placed around the perimeter of the enclosure at or slightly above the discharge trough of the spout. The header typically contains spray nozzles or holes drilled in the header and spaced uniformly around the perimeter to yield a uniform curtain of wash water that keeps the lower portion of the enclosure (known as the “skirt”) wet and washed. The preferred cleaning medium is weak wash because its use does not disturb the mill's liquor cycle balance. However, the solids in the weak wash have a tendency to plug the holes in the wash header reducing the coverage and effectiveness of the washing system.
The operation and maintenance of the enclosure suffers when the skirt washing header and its nozzle(s) plug and materials are allowed to accumulate on skirt walls. Aside from the return of buildups to the unwashed area, several other issues tend to occur. The dry zone tends to have a higher temperature than the washed areas where thermal differential expansion buckles the skirt walls. This buckling disturbs the sheeting action of the original wash system and if allowed to continue, this area will not be properly washed again until the skirt walls are straightened. Another impact of this condition is that locally higher temperatures can accelerate corrosion in the skirt walls. Tramp air ensues and either distorts the flow of smelt from the spout which tends to re-oxidize the smelt (lowering the reduction efficiency) or can overload the capacity of the scrubber vent fan which may allow gases to escape the dissolving tank into the work environment.